Some image acquisition devices, such as digital cameras and smartphone cameras, include an autofocus (“AF”) optical system operable to focus on a selected point or area of a scene. The AF feature can provide the device with the ability to focus automatically on a subject correctly, without the need for manual intervention from the user.
Various types of AF control are available, including passive control (e.g., contrast AF), and active control (e.g., Laser AF). Smartphones, for example, tend to use passive AF, because contrast is needed for the lens to focus. Such devices may have difficulty focusing on blank colored surfaces or in low light. A flash, which is included on some smartphones, can help produce artificial light to aid in the focusing in these situations. Contrast AF tends to be highly iterative and thus can be time-consuming and can use significant computational resources. Active AF, on the other hand, involves actively measuring the distance to an object in a scene, and then adjusting the optics for proper focus of that particular object. Active AF often works well in a low-light environment, but only a limited area typically is illuminated by the active illumination. Consequently, distance information of a limited area in a scene is known. If a user desires another area in the scene to be in focus, the camera or host device must retake the distance measurement.